The derivation of clinically relevant human embryonic stem cells (hESCs) from human blastocysts represents one of the milestones in stem cell biology [1]. hESCs have the capacity for extensive self-renewal under in vitro culture conditions. A second hallmark of these cells is their ability to undergo multi-lineage differentiation; also defined as pluripotency. Hence, the robust self-renewal capability of these pluripotent hESCs makes them a renewable source for the generation of functional cell-types or tissues for potential therapeutic applications and drug discovery. Importantly, hESCs provide an opportunity to study early human developmental biology—an area of study where it is difficult to acquire experimental data.
In addition to the ability to self-renew and differentiate, hESCs share many similarities with mouse embryonic stem cells (mESCs) [2, 3]. Both of them express genes which are associated with pluripotency [4-6]. POU5F1 (coding for the protein OCT4) and NANOG, both key components of the core transcriptional regulatory network [7-9], are highly expressed in undifferentiated ESCs [10-15] and upon differentiation, the expression of these genes is reduced. These and other transcription regulators, including the co-activator p300, show extensive co-localization at genomic sites and this binding configuration may be important for the expression of pluripotency-specific genes [9, 16, 17].
However, there are significant and intriguing differences between hESCs and mESCs. One of the differences is the signaling pathways that promote ESC identity. The fibroblast growth factor/Mitogen-Activated Protein Kinase Kinase (FGF/MEK) pathway is important for the propagation of hESCs. In contrast, stimulation of the FGF/MEK pathway promotes differentiation of mESCs. The inhibition of FGF signaling using specific FGF receptor/MEK inhibitors, in combination with glycogen synthase kinase-3 inhibitor, allows mESCs to be propagated in the absence of other growth promoting molecules [18]. The TGFβ/Activin/Nodal pathway maintains the self-renewal of hESCs but not mESCs, through the up-regulation of NANOG expression [19, 20]. Leukemia inhibitory factor (LIF) and bone morphogenetic protein 4 (BMP4) are known to sustain mESCs, but LIF does not support hESCs and BMP4 induces hESCs to differentiate [4]. Moreover, hESCs and mESCs are morphologically distinct where mESCs form dome-shaped colonies while the hESC colonies are flat. In addition, certain surface molecules like SSEA-3 and SSEA-4 are present in undifferentiated hESCs but not mESCs [4].
The differences between both mouse and human ESCs could be due to species-specific differences in embryonic development. Alternatively, the ESCs could be derived from cells originating from different developmental stages. Consistent with this idea is the identification of post-implantation murine epiblast-derived stem cells which show characteristics of hESCs [21, 22]. Hence, it is useful to understand the differences between these ESCs and the molecular basis for the differences. Furthermore, to harness the full potential of hESCs, it is valuable to dissect the mechanisms that maintain the identity of hESCs.